Membrane technology includes process engineering measures for the transport of substances between two fractions with the help of permeable membranes. It can find practical applications in membrane system operations involving membrane process, such as but not limited to Reverse Osmosis (RO), Ultra Filtration (UF), Microfiltration (MF), etc.
Raw seawater contains sparingly soluble salts, such as carbonate scale, sulfate scale, bio-scale and so forth which have very low solubility thereby constituting a very high specific potential for scaling on membrane surface of the permeable membranes used in the membrane system operations. The concentration of these salts in reject water increases with the extent of removal of product water. The high concentration of low solubility salts can cause precipitation and crystallization of salts on membrane surface. This phenomenon is known as membrane scale formation. Similarly the organic salts and microorganism in the feed water also gets deposit on the membrane surface and on the spacers, and this phenomenon is known as membrane fouling. Membrane fouling/scaling causes a higher energy use and a shorter life span of the membranes, because of the more often need for cleaning.
Inorganic salts such as calcium carbonate, barium sulfate and so forth which are water insoluble can become oversaturated, causing them to precipitate. Hence the feed water to membrane process is treated with scaling control chemicals to avoid precipitation of these salts on the membrane surface. These scaling control chemicals are called as antiscalants. The antiscalants adsorb onto growth sites of the scales and prevent further growth, crystallization and precipitation of scalants. The type of antiscalant chosen governs the extent of diffusion kinetics reaction between the scalant and antiscalant. Also, in order to avoid deposition of organic content on the membrane surface, the feed water is treated with alkaline solution as antiscalant. Hence the choice of antiscalant and its dosage can be very important in scaling/fouling control of membranes for better membrane performance and operation.
Currently, the type of antiscalant used and its dosage are determined based on recommendations from membrane manufacturers, chemical suppliers, consultants and the feed water quality. The scale deposited in the membrane could be predominantly either carbonate scale, sulfate scales, bio-scale and so forth, depending on the feed water quality. In industrial applications, antiscalant is suggested based on the type of component which is dominantly present in the feed water. For example, sodium hydroxide is suggested for sulfate scales and organic fouling, hydrochloric acid is suggested for carbonate scale and so forth. The antiscalant recommended will dissolve only that particular component and may not dissolve all the components present in the scale. Hence, it may be desired to use a mixture of antiscalants with a particular composition, in order to dissolve and remove all the components present in the scale.
The antiscalants are dosed along with feed water to maintain their concentration at a specific value in the reject stream. Since some of the antiscalants are not measurable by available sensors in the market, a fixed excess amount of antiscalants are dosed to prevent membrane scaling and subsequent damage. This technique is based on a rule of thumb and can result in excess use of antiscalants which are highly expensive, and does not consider the current condition or status of a membrane to estimate the amount of antiscalants to dissolve existing scale precipitates and crystals at a membrane surface. It should be noted that, even though in practice a fixed excess amount of antiscalants are dosed, the amount of antiscalant present in the product water and reject stream should be within allowable limits as recommended by World Health Organization (WHO) such that it does not involve any post-treatment operations to remove these antiscalant chemicals.
The present disclosure includes methods for choosing proper antiscalants and optimal dosages. The membrane cleaning chemistry and its kinetics can be vital in deciding the proper choice of antiscalant while the dosage of antiscalants is estimated based on membrane fouling status.